Methods for seabed piston coring

ABSTRACT

A method of acquiring a core sample of seabed material into a core sampling tube having an upper end, a lower open end and a substantially cylindrical chamber extending therebetween, comprising the steps of urging the core sampling tube into the seabed and simultaneously withdrawing fluid from the upper end of the core sampling tube at a rate sufficient to cause the seabed material to be drawn into the core tube at substantially the same rate as the core tube penetrates the seabed.

FIELD OF THE INVENTION

The invention relates to technology used for taking core samples fromthe seabed using a drill that is lowered and controlled remotely from aship.

BACKGROUND OF THE INVENTION

Conventionally taking core samples from the seabed has been achieved byeither a technique known as piston coring or diamond coring.

Diamond coring is achieved by using conventional core barrels withdiamond set bits. Commonly this technique is used when drilling rock.

On the other hand piston coring is particularly suited to seabedoperations where typically the seabed is covered with a layer ofsedimentary material that is too soft to core successfully usingstandard diamond coring system.

The current invention relates to improvements in this latter method andtherefore the following description deals in detail with that type ofprior system.

It is well known to take short samples with core sampling tubes such asthe Shelby tube However, it has been found that the friction on thesample acting on the inner walls of the tube quickly builds up toprevent the entry of new material. This means that the tube becomeseffectively a solid rod and displaces the sediment without any furtherwinning of sample.

This effect is particularly damaging when there are layers of very softand harder material, as the friction of the harder material preventsany, or at most little, of the soft material entering the tube. Thesample in the tube then consists almost entirely of the harder material.

Other conventional sampling techniques for the seabed take advantage ofthe water pressure at depth to take longer and more representativesamples by use of tethered piston coring technology. In such technologythe drill frame located near the seabed by support means and includes ahydraulic feed cylinder and rope and pulley system. The feed cylindercauses the core sampling tube to be pushed into the seabed. A piston isinstalled inside the sampling tube and includes seals to prevent leakagepast the piston. The piston is supported from the frame by tether rope,so that, as the tube is pushed into the seabed, the piston isconstrained to remain stationary.

If the friction of the material in the tube creates enough force toovercome the hardness of the material entering the bottom of the tube,the material in the barrel will try to move down with the tube.Providing that the material is essentially impervious, this will createa reduced pressure under the tethered piston. The difference in pressurebetween that at the bottom of the tube and that under the piston is thenavailable as an additional force to overcome the friction of thematerial in the tube.

The reduced pressure under the piston is self-regulating as it isgenerated by the friction in the tube and the pressure gradient down thetube is proportional to the friction in each part of the tube. Thismeans that a complete sample of the seabed is obtained, complete withsoft and hard layers.

It will be apparent that this process becomes more effective withincreasing water depth because the available reduction in pressureincreases. It is essentially ineffective on or near the surface.

Whilst this system is effective, it has been difficult to apply thismethod to a drill that has a segmented drill string made up of avariable number of drill rods depending on penetration depth, becausethere is no practical way of connecting the tether rope to the piston inthe core barrel at the bottom of the drill string.

Accordingly further investigations have been carried out in attempt toimprove the applicability of a piston based coring system.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome the limitations ofcurrent piston coring systems, more particularly, to obviate the need touse a structurally tethered piston.

SUMMARY OF THE INVENTION

Accordingly in one aspect of the invention, a method of acquiring a coresample of seabed material into a core sampling tube having an upper end,a lower open end and a substantially cylindrical chamber extending therebetween, comprising the steps of urging the core sampling tube into theseabed and simultaneously withdrawing fluid from the upper end of thecore sampling tube at a rate sufficient to cause the seabed material tobe drawn into the core tube at substantially the same rate as the coretube penetrates the seabed.

Preferably, the step of withdrawing fluid from the upper end of the coresample tube comprises withdrawing the fluid through a conduit meansconnected at one end to the core sampling tube and connected at itsother end to a remote means for withdrawing fluid. Preferably, the stepsof urging the core sample into the seabed and withdrawing fluid fromabove the seabed material is performed by a combination of remotelycoordinated hydraulic fluid power means. Typically, the coordination ofthe hydraulic fluid power means comprises the steps of pumping hydraulicfluid into a first hydraulic means to urge the core sampling tube intothe seabed and simultaneously pumping hydraulic fluid into a secondhydraulic means to withdraw fluid from the upper end of the coresampling tube.

It will be understood that a freely movable piston may or may not belocated in the core sampling tube. It will be included where there is asignificant risk that seabed material may also be withdrawn from thesampling tube.

Accordingly, it is preferred to provide the core sampling tube furtherwith a piston sealingly engaging and movable within the cylindricalchamber above the seabed material entering the core tube, and the stepof withdrawing fluid is from above the piston such that the piston ismaintained substantially stationary.

In a separate aspect of the invention which is adapted to be used withthe method described above, a core sampling tube is provided comprisinga core barrel having an upper end with a fluid inlet/outlet, an openlower end and a substantially cylindrical chamber extending therebetween to receive seabed material.

Preferably, the core sampling tube further comprises a piston sealinglyengaging the cylindrical chamber and movable axially within thecylindrical chamber in response to fluid flow through the inlet/outlet.

Preferably, the core sampling tube further comprises an adaptation atthe upper end to provide sealing means to permit a leak free connectionto the conduit connectable between the core sampling tube and the remotemeans for withdrawing fluid.

In a further separate aspect of the invention which is adapted to beused with the method and core sampling tube described above, a seabedcoring system is provided comprising:

(a) a core sampling tube described above;

(b) first hydraulic fluid power means to urge the core sampling tubeinto the seabed;

(c) second hydraulic fluid power means to withdraw fluid from the coresampling tube above the seabed material; and

(d) first conduit means connected between the core sampling tube and thesecond hydraulic fluid power means;

 wherein the first hydraulic fluid power means and the second hydraulicfluid power means are coordinated such that the seabed material willenter the core sampling tube at substantially the same rate as the coretube penetrates the seabed.

Preferably, the seabed coring system further comprises apiston,sealingly engaging and movable within the cylindrical chamber ofthe core sampling tube above the seabed material entering the core tube.

Preferably, the first hydraulic fluid power means comprises asubstantially cylindrical chamber, a piston sealingly engaging thecylindrical chamber and movable axially within the cylindrical chamberto define a first chamber and a second chamber, and a piston rodconnected to the piston and extending through and from the secondchamber so that selective application of hydraulic pressure to the firstchamber will urge the core sampling tube into the seabed.

Preferably, the second hydraulic fluid power means comprises:

(a) a first sub hydraulic means including a substantially cylindricalchamber, a piston sealingly engaging the cylindrical chamber and movableaxially within the cylindrical chamber to define a third chamber and afourth chamber, a piston rod connected to the piston at one end thereofand extending through the fourth chamber;

(b) a second sub hydraulic means comprising a substantially cylindricalchamber, a piston sealingly engaging the cylindrical chamber and movableaxially within the cylindrical chamber to define a fifth chamber, thepiston rod of the first sub hydraulic means having its other endconnected to the piston; and

(c) second conduit means connected between the second chamber of thefirst hydraulic means and the fourth chamber of the first sub hydraulicmeans;

 wherein, as the core sampling tube is urged into the seabed by thefirst hydraulic fluid power means, hydraulic fluid is passed from thesecond chamber of the first hydraulic fluid power means into the fourthchamber of the first sub hydraulic means via the second conduit means tomove the piston therein which in turn draws the piston of the second subhydraulic means away from the first conduit means to cause thewithdrawal of fluid from the core sampling tube.

Typically, the first conduit means consists in part of at least one hosewith high collapse capability.

In another typical arrangement, the first conduit means consists in partof at least one drill rod with sealing means to provide a leak freejoint between the drill rod and any preceding drill rod.

It will be appreciated that three separate aspects of the invention havebeen disclosed. namely, a method of acquiring a core sample from aseabed, a core sampling tube and a system (apparatus) for acquiring acore sample. Whilst the description explains preferred embodiments ofeach aspect in combination with one another, such aspects are not sointerdependent and should not be so construed.

DESCRIPTION OF THE DRAWINGS

The invention will now be further illustrated with reference to theaccompanying drawings in which:

FIG. 1 is an embodiment of a type of drill useable with the invention.

FIG. 2A is a plan view of a drill useable with the invention.

FIG. 2B is a side view of the drill of FIG. 2A.

FIG. 3 is a more detailed side view of the drill of FIG. 2A.

FIG. 4 is an end view of the drill of FIG. 2A.

FIG. 5 is a more detailed plan view of the drill of FIG. 2A.

FIG. 6A is a side view of the rotary drilling unit.

FIG. 6B is a plan view of the rotary drilling unit of FIG. 6A.

FIG. 7 is a side sequential view of the drilling equipment.

FIG. 8 is a side view of the drilling procedure.

FIG. 9 is an expanded side view of the rod and casing clamp area.

FIG. 10 is a cross-sectional view showing part of the water circuit forrock coring.

FIG. 11 is a schematic representation of the principle of operation ofpiston coring according to the prior art.

FIG. 12 shows in schematic, a preferred embodiment of a method of pistoncoring according to the invention.

FIG. 13 is a cross-sectional view of the sealed drill string for pistoncoring according to the invention.

FIG. 14 is a hydraulic circuit used with piston coring according to theinvention.

FIGS. 15A to 15F depict the sequential operation of a piston core barrelin accordance with the invention.

FIG. 16A is a cross-sectional view of the initial position of analternate form of core barrel in accordance with the invention.

FIG. 16B is a cross-sectional view of the final position of thealternate form of core barrel of FIG. 16A.

FIG. 17 is a cross-sectional view of the initial position of a furtheralternate form of core barrel in accordance with the invention.

Geological samples on land are often obtained using core drills,typically with diamond tipped drill bits. Similar drill rigs can bemounted on ships and used to take core samples from the seabed, but withgreater difficulty as ships move with the sea surface and the water canbe very deep. The drill string has to go through the water column beforereaching the seabed. The provision of a ship of adequate size capable ofholding position with sufficient accuracy adds considerably to the cost.

In recent years, drills capable of sitting on the seabed have beendeveloped as they provide a more stable drilling platform and can beused with a less sophisticated and cheaper ships.

FIG. 1 shows a typical deployment of a seabed drill. A suitable ship 1-1has carried the drill to the site, swung it over the stem using anA-frame 1-2 and lowered it to the seabed with a winch mounted on thedeck of the ship.

The drill is powered by one or more electric motors driving hydraulicpumps so that all mechanical operations are carried out hydraulicallythrough the use of hydraulic motors, rotary actuators and cylinders asappropriate. The drill is remotely controlled from the ship as it isusually deployed in water depths beyond those accessible to a diver.Essential functions are monitored with appropriate remote sensingdevices such as pressure switches, pressure transducers and proximitysensors. Undersea video cameras are used to provide visual feed-back.

The cable 1-3 is preferably of a multi-purpose type with steel outerlayers to provide the required lifting capability and coveringelectrical conductors to provide the power for the drill and afibre-optic core for control and telemetry. However, it is possible touse a normal wire cable for lifting, with power and communicationsachieved by a separate bundle of cables, typically incorporating floatsalong its length to achieve neutral or slightly positive buoyancy.

The float 1-4 holds any cable slack away from the drill and acts toisolate the drill from movement of the ship due to sea swell and waves.

The drill itself 1-5 sits firmly on the seabed under the action of itsown weight on legs 1-6, possibly assisted by suction feet. Details ofthe drill construction will be discussed late in this specification.

The location of the drill is established by reference to acoustictransponders mounted on the drill on the ship and on marker buoys 1-7.Acoustic receivers on the drill and on the ship provide triangulatedpositioning information.

The following description is of a particular design of drill of theseabed type, but it will be understood that the invention is not limitedto use with such types of drills.

The basic operation is that the drill is lowered to the seabed withenough empty sampling tools to acquire the penetration desired,typically less than 100 m, and with sufficient drill rods to place thesampling tools to depth, and sufficient casing to hold the hole open aseach sampling tool is removed and stored back on the drill. The drillcan be loaded with different combinations of several types of samplingand ground testing tools, drill rod and casings to suit the particularconditions of the seabed being investigated.

Typically, the drill tools are 3 m long, giving a total drill height ofaround 5 m with a total weight of about 7 tonne.

FIGS. 2A and 2B shows a plan view at the top, and side elevation of aseabed drill consisting of the main body of the drill 2-1 and three legs2-2 with feet 2-3. The elevation shows one leg 24 fully extended byhydraulic cylinder 2-5 and another leg 2-6 fully retracted to its stowedposition and with its foot removed.

The legs are retracted to stowed position for lifting on and off theship, and the feet are removed for transport from ship to ship. The feetcan be made in the form of suction cans and connected to a source ofreduced water pressure, such as the suction of a water pump, effectivelysucking the feet onto the bottom, to provide a positive hold-down forthe drill so that its stability may be increased beyond that obtainedfrom its own weight in water.

FIG. 3 shows a more detailed side elevation of the drill, illustratingmany of its main components. This drill is designed for penetrationdepths of 100 m and requires that the drilling tools be stored in rotarymagazines 3-1. In this case there are two magazines, one normally usedfor core barrels and a second for drill rods and casing. Simpler drillsfor very shallow penetration may have only a single drill tool andrequire no storage.

The multi-purpose lift/power/control cable 3-2 passes through a topguide 3-3 to an anchor point 3-4 at the drill base. The powerconductors, not shown, are connected to electric motors 3-5 which drivehydraulic pumps 3-6 which power all the mechanical functions of thedrill through hydraulic control valves and actuators not shown.

Drilling tools are picked up from the magazines by loading arms 3-7 andpresented to the drilling centre line, where they are picked up by therotary drilling unit 3-8 which is mounted on vertically sliding carriage3-9. The rotary drilling unit is described in more id detail later. Thecarriage is moved up and down the elevator mast 3-10 on slides 3-11 by ahydraulic cylinder with a 2:1 rope and sheave system not shown.

A rod clamp 3-12 and casing clamp 3-13 are mounted in the base frame.

FIG. 4 shows an end elevation of the drill. This view shows that thisdrill design has two storage magazines, 4-1 and 4-2, and that each isrotated by a Geneva wheel pinion 4-3. The Geneva wheels themselves 4-4are not shown in plan, but have the same number of slots as themagazine, see later, so that each full rotation of the pinion advancesthe magazine one complete slot.

FIG. 5 shows a more detailed plan view of the drill. The Geneva drivepinions 5-1 independently driving the two magazines 5-2 are shown withthe magazine top swivel bearings 5-3.

A plan view of the loading arms 5-4 shows the double jaw structure. Theloading arm is pivoted by rotary actuator 5-5 to move drilling toolsbetween the magazines and the drill centre line 5-6 as required for thedrilling process.

A plan view of the rotary drilling unit 5-7 is visible partly occludedby the top structure. The spooling drum 5-8 holds the hoses and cablesthat are connected to the rotary drilling unit and is moved up and downat the same time as the rotary drilling unit to keep the hoses andcables organized.

The top sheaves 5-9 are part of the 2:1 cable system on the carriageelevator.

One of the alignment guide arms 5-10 is shown. The other arm issymmetrical with the one shown and on the other side, under the loadingarm. They are both operated by hydraulic cylinders to swing into thecentre to clamp onto a drill tool in position on the drill centre line.

FIGS. 6A and 6B show more details of the rotary drilling unit which ismounted to the carriage by means of pins and bolts through lugs 6-1. Thedrive power is provided by a hydraulic motor 6-2 driving though agearbox 6-3 which provides both a gear reduction and an off-set drive.

The output of the gearbox drives the rotating chuck 6-4 which isoperated hydraulically through a hydraulic slip ring in stationarycentre housing 6-5.

A hydraulically operated rack drive system for breaking out drill toolthreads is enclosed in housing extension 6-6. This rack system engagesthe output gear of the gearbox to provide a direct high reverse torque.

The output shaft of the gearbox also protrudes through the top of thegearbox, and is hollow, connecting the top to the inside of the rotatingchuck. A rotary swivel coupling 6-7 is mounted on the top of the shaftfor water connection to the drill string.

FIG. 7 shows the main components used during the drilling process. 7-1is the rotary drilling unit just described. The upper and lower loadingarms, 7-2 and 7-3 respectively, which will be described in more detaillater, fetch tools from the magazines and return them after use.Alignment guide 7-4 and alignment guide spacer 7-5, again described inmore detail later, assist in the thread make-up between drill tools.

Rod clamp 7-6 is hydraulically operated and similar in design to thehydraulic chuck on the rotary drilling unit. It is used to hold thedrill string while a tool is added or removed from the string.Intermediate guide 7-7 provides location for the drill casing whichcontributes to the positioning of the drill on the seabed. Casing clamp7-8 is identical in construction to the rod clamp but is used to clampthe drill casing string. Bottom guide 7-9 also provides location for thecasing, in conjunction with the intermediate guide and casing clamp.

The bottom of the drill 7-10 is normally positioned on or near to theseabed by adjustment of the drill legs.

FIG. 8 illustrates a part of a typical coring cycle. Each core sample istaken and stored in a separate core barrel. For each successive samplean empty core barrel is introduced into the hole and lowered down to theprevious finish depth by adding the required number of drill rods to thedrill string. The sample is then taken and the core barrel withdrawn, bysequentially removing the drill rods, and stored back in the magazine.This process is repeated into the deepening hole until the requiredmaximum sample depth is achieved.

Casings can be installed separately, but in similar manner, as required.

The sequence shown on FIG. 8 starts at step A with a first core samplealready taken, and a length of casing 8-1 subsequently installed andheld in casing clamp 8-2. A core barrel 8-3 taken from a magazine andpresented to the drill centre line by loading arms 8-4.

In step B, the rotary drill unit 8-5 has been lowered and its chuckgrabbed the top of the barrel. The alignment guide 8-6 locates thebottom of the barrel. The alignment guide spacer 8-7 is deployed to holdthe guide slightly open so that it does not clamp on the barrel, butmerely provides a sliding guide. Once the barrel is held, the loadingarms are moved out of the way.

As the barrel is lowered into the hole by the rotary drill unit, thealignment guide is withdrawn. Step C shows the barrel lowered to thebottom of the hole where it is clamped by the rod clamp 8-8. The rotarydrill unit then retracts to its top position in Step D and a drill rod8-9 is brought into the centre line by the loading arms.

Step E shows the drill rod held by the chuck of the rotary drill unit atthe top and by the alignment guide at the bottom. The alignment guidespacer is retracted so that the alignment guide clamps onto the top ofthe core barrel to provide guidance for thread make-up.

The rotary drill unit then lowers and rotates to make up the threadbetween the drill rod and core barrel. The alignment guide then retractsas shown in Step F.

Step G shows the core barrel at its full depth having taken the nextsample. This is then withdrawn from the hole by the reverse of thesequence described above and stored back in the magazine.

The next operation would typically be to install a new length of casingto the new depth, followed by another core barrel to the next depth.

FIG. 9 shows an expanded view of the clamp area in Step E of FIG. 8. Thecasing 9-1 is supported by the bottom guide 9-2 and intermediate guide9-3 and is held by casing clamp 9-4.

During diamond core drilling, the rock cuttings from the drillingprocess normally pass up the inside of the casing and exit at the top ofthe casing into gallery 9-5 formed in the intermediate guide. Thesuction of a suitable centrifugal pump is connected to outlet 9-6 toremove the cuttings from the clamp area and discharge them into a piperunning along one of the drill legs.

The rod clamp 9-7 is shown holding a core barrel 9-11 with the alignmentguide 9-8 deployed to clamp around the top of the barrel. A drill rod9-9 is shown ready to engage its thread with a mating thread in the tipof the barrel. The alignment guide spacer 9-10 is shown in retractedposition. It is operated by a small hydraulic cylinder, not shown.

One known method of coring is diamond coring using diamond set bits.This equipment is commonly used for rock coring on land and theoperation of this device will be well known to those skilled in coredrilling.

For operation, the drill has to provide rotation and downward force in acontrolled manner so that the diamond bit at the bottom cuts its wayinto the rock.

A supply of water is provided through the hollow drill rods to the topof the core barrel and discharges with the cuttings, up the outside ofthe barrel.

This water is supplied by a water pump, driven by a hydraulic motor,mounted on the drill. The delivery from this pump connects with aflexible hose to the rotary drilling unit to accommodate its verticalmovement.

FIG. 10 shows a part sectional view of a rotary drill unit. Hollow shaft10-1 is supported within the housings 10-2 on bearings, not shown, androtated by hydraulic motor 10-3 through gears, also not shown. Driveplate 104 is attached to the hollow shaft and supports chuck assembly10-5. One of three chuck cylinders is shown 10-6 with chuck jaw 10-7.The chuck cylinder is connected through conduits 10-8 to a slipringincorporated in the hollow shaft.

The drill water supply is delivered into flexible hose 10-9 throughrotary coupling 10-10 into the centre of the hollow shaft, then throughseal piece 10-11 which seals against the end of the drill rod 10-12,which has a hole, not shown, through its length. This drill rod may beconnected to other drill rods to make up the drill string, depending onthe drill depth, or to the core barrel 10-13 as shown.

The core barrel drills a slightly oversize hole so that the water canflow on the outside of the barrel and then past the drill rod and out ofthe top of the hole.

Another known coring system is piston coring. Much of the seabed iscovered with a layer of sedimentary material that is too soft to coresuccessfully using standard diamond coring systems as just described.

Short samples can be achieved using conventional soil samplingtechniques such as the Shelby tube, but the friction on the sampleacting on the inner walls of the tube quickly builds up to prevent theentry of new material, so that the tube becomes effectively a solid rodand displaces the sediment without any further winning of sample.

This effect is particularly damaging when there are layers of very softand harder material, as the friction of the harder material preventsany, or at most little, of the soft material entering the tube. Thesample in the tube then consists almost entirely of the harder material.

Conventional sampling on the seabed takes advantage of the waterpressure at depth to take longer and more representative samples by useof piston coring technology. FIG. 11 shows a schematic of a pistoncoring system. A drill frame 11-1 is held near the seabed by supportmeans not shown and includes a hydraulic feed cylinder 11-2 and rope andpulley system 11-3, so that extending the feed cylinder causes the coresampling tube 11-4 to be pushed into the seabed. A piston 11-5 isinstalled inside the sampling tube and includes seals to prevent leakagepast the piston.

The piston is supported from the frame by tether rope 11-6, so that, asthe tube is pushed into the seabed, the piston is constrained to remainstationary.

If the friction of the material in the tube creates enough force toovercome the hardness of the material entering the bottom of the tube,the material in the barrel will try to move down with the tube.Providing that the material is essentially impervious, this will createa reduced pressure under the tethered piston. The difference in pressurebetween that at the bottom of the tube and that under the piston is thenavailable as an additional force to overcome the friction of thematerial in the tube.

The reduced pressure under the piston is self-regulating as it isgenerated by the friction in the tube and the pressure gradient down thetube is proportional to the friction in each part of the tube. Thismeans that a complete sample of the seabed is obtained, complete withsoft and hard layers.

Referring again to FIG. 11, the seabed is shown as two layers, with ahigh friction layer 11-7, perhaps stiff clayey sand, overlaying a lowfriction base 11-8 of say mud.

The graph 11-9 shows the distribution of reduced pressure down theinside of the tube. The lowest pressure 11-10 is just under the piston,the pressure gradient 11-11 through the high friction material issteeper than the gradient 11-12 through the low friction material. Thepressure at the mouth of the tube is substantially equal to the ambientpressure at that water depth.

This process becomes more effective with increasing water depth becausethe available reduction in pressure increases. It is essentiallyineffective on or near the surface.

It is difficult to apply this method to a drill that has a segmenteddrill string made up of a variable number of drill rods, depending onpenetration depth, because there is no practical way of connecting thetether rope to the piston in the core barrel at the bottom of the drillstring.

FIG. 12 shows a schematic of a method of applying the same principles ofoperation without the use of a mechanical tether for the piston. Thedrill frame 12-1, hydraulic feed cylinder 12-2, rope pulley system 12-3and core sampling tube 12-4 remain the same as described with FIG. 11.

In this case the tether rope is not used, but the chamber 12-6 above afloating piston 12-5, being filled with water, is connected by conduit12-7 to water cylinder 12-8. The piston 12-9 is operated by a secondhydraulic cylinder 12-10, called the coring cylinder, which isinterconnected to the feed cylinder by connection 12-11.

The water cylinder and coring cylinder are sized so that extension ofthe feed cylinder to push the core tube into the seabed causesretraction of the coring cylinder, drawing water into the water cylinderso that the floating piston is drawn into the core tube at the same rateas the core tube penetrates the seabed. By this means the floatingpiston is held a stationary relative to the seabed, thus providing thesame method of core sampling as is achieved with the mechanicallytethered system.

The floating piston has low friction so that there will be substantiallyequal pressures above and below the piston. The pressure in conduit 12-7is thus a direct measure of the frictional resistance of the materialbeing sampled, so that the use of a pressure transducer, for example,provides information on the sediment characteristics.

The same result can be achieved without the floating piston at all withthe material in the tube effectively acting as the piston, but the useof a piston is preferred as it minimises disturbance to thewater/sediment interface and prevents the sample being inadvertentlydrawn up into the conduit.

The combination of components described above is called a “hydraulictether” system as it replaces the conventional mechanical piston tether.

The conduit 12-7 as applied to the seabed drill passes through a numberof components as will be described with reference to FIG. 13.

FIG. 13 is similar to FIG. 10 used for rock coring but with someimportant differences. The rock core barrel is replaced with a pistoncore barrel 13-1 incorporating a sealed piston 13-2. The connection withthe drill rod 13-3 now has a seal 13-4 to ensure a leak free joint withexternal pressure higher than internal pressure. Any leakage wouldreduce the effectiveness of the hydraulic tether system. If there is anumber of drill rods, there will be similar seals at each join.

Similarly, the top of the drill string is sealed 13-5 in the chuckassembly.

As the drill will be used for both rock drilling and piston coring, therotary coupling 13-6 has to withstand both moderate internal pressureand potentially higher external pressure., depending on water depth andsediment friction characteristics. Similarly the hose 13-7 has towithstand a high external collapse pressure.

As the drill will be used for rock coring as well as piston coring, thedrill water has to be valved to either the drill water pump or thehydraulic tether system, achieved by the use of conventional poppetvalves operated by small hydraulic cylinders, not shown.

FIG. 14 shows a part of the oil hydraulic circuit illustrating therequirements for engagement of the hydraulic tether system.

In the position shown the feed cylinder 14-1, refer also 12-2, is heldstationary by the closed centre of proportional solenoid valve 14-2. Ifsolenoid b of this valve is energised, the feed cylinder will beextended, with the return flow from the rod end directed to returnthrough over centre valve 14-3. The over centre valve acts to hold theweight of the rotary drill unit, carriage and drill string so that thelowering speed is controlled by the oil feed into the feed cylinder.Check valve 14-10 prevents flow back to return though mode selectionsolenoid valve 14-4 when it is in the neutral position shown.

If solenoid a is energised the feed cylinder is retracted, causing thedrill string to be raised.

The mode selection valve provides additional functionality by selectingthe destination of the return flow from the rod end as the feed cylinderextends. With solenoid b of the mode selection valve, the return flow isconnected back into the feed cylinder to provide a regenerative effectfor faster cylinder operation. Check valve 14-5 prevents the return flowpassing back through the proportional valve. Counterbalance valve 14-6acts to hold the weight in the same manner as the over centre valve.

Energising of solenoid a of the mode selection valve directs the returnflow from the rod end of the feed cylinder to the coring cylinder 14-7,refer also 12-10, so that the coring cylinder is retracted at a speedproportional to the speed of extension of the feed cylinder, with aratio depending on their relative piston and rod sizes. The coringcylinder then operates the water cylinder as described with reference toFIG. 12. Over centre valve 14-3 now acts as a pressure relief valve tolimit the maximum pressure to the coring cylinder.

Coring reset solenoid valve 14-8 is used to return the coring cylinderto the retract position after the piston coring process. The orifice14-9 limits the reset speed.

The hydraulic tether system can be used with a range of coring toolswith two preferred embodiments described in the following drawings.

FIG. 15A shows a piston core barrel 15-1 in a casing 15-2 ready to takeanother in a series of samples. The casing has a bit 15-3 that allows itto ream out the hole as it is advanced, described in more detail later.The core barrel has a cutting edge 15-4 incorporating a segment typecatcher 15-5 attached to the bottom of core barrel tube by means notshown, but typically a press fit, or small grub screws or rivets. Afloating piston 15-6 starts at the bottom of the tube as shown, in thiscase positioned by the lip of piston seal 15-7 catching on the edge ofthe top of the cutting edge assembly. It could be positioned by othermeans such as a spring retaining ring.

A liner 15-8, typically plastic, is fitted to the majority of the lengthof the barrel. A washer 24-9 is positioned at the top of the liner whichis used to assist in extracting the sample from the barrel when thedrill is unloaded when back on board ship. After removal of the cuttingedge and catcher, the washer is pushed down by a suitably sized rod,which then pushes the sample and liner out of the tube. The sample isnormally left in the tube and either cut along its axis to split thesample into longitudinal halves or into shorter lengths for testing andother investigations.

Check valve 15-10, which can be removed for the sample extractiondescribed above, allows water to pass out of the barrel but then acts toprevent the floating piston going back down again.

Drill rod 11 is shown attached to the top of the barrel, ready to pushthe barrel into the sediment.

In operation, the hydraulic tether system is connected and the barrelpushed down. The tether system holds the floating piston stationary bydrawing water out of the barrel through the check valve. As the tubeextends down over the piston, the seal engages inside the liner toproduce a leak proof seal.

The barrel is pushed down quickly, typically a few seconds for the wholelength, because the effectiveness of the hydraulic tether system isdependent on the low porosity of the material being sampled, so thatfaster operation allows successful sampling of materials with somedegree of porosity. Normally the speed of operation is limited by theoutput of the hydraulic pumps acting on the feed cylinder, but fasteroperation can be achieved, about one second, by the use of energy storedin a differential hydraulic accumulator.

FIG. 15B shows the barrel fully extended, now full of sampled sediment15-17, with the floating piston 15-6 now close to the top of the barrelin the same position as in FIG. 15A.

The hydraulic tether pressure would be recorded during this process sothat the performance can be monitored. The actual pressure change duringpenetration provides information on the friction characteristics of thematerial. The pressure should rise progressively during the penetrationswith a pressure plateau indicating that the material is too porous for acomplete sample to be obtained, that water has flowed through thematerial to collect under the piston. A sudden rise in pressure mayindicate that the piston has reached the end of its stroke for somereason.

The barrel is now pulled out and stored back on the drill. The sampledsediment 15-17 is held in the barrel, see FIG. 15C, by the combinedaction of the segmented catcher 15-5 and the check valve 15-10preventing the piston 15-6 from moving down the tube. The material belowthe catcher 15-12 may fall out and be lost, or may remain due to its ownfriction and suction.

FIG. 15D shows the hole left behind after the barrel is removed.Commonly the hole would slump due to the softness of the material withloose material 15-13 filling the bottom of the hole and a void 15-14appearing at the top.

The casing is now advanced to the bottom of the hole, using feed down,rotation and drilling water. Normally this operation will flush theloose material out of the hole, up the outside of the casing with thedrill water discharge, but sometimes this will be ineffective so thatthere is still loose material 15-15 inside the casing as shown on FIG.15E. This occurrence will usually be apparent by the lack of drill waterflow during the process of setting the casing.

In this case, a cleaning out tool 15-16, FIG. 15F, can be deployed toclean out the hole to the bottom of the casing. The hole is now readyfor the next core barrel, starting again as in FIG. 15A.

FIGS. 16A and 16B show another type of piston core barrel that can usedwithout casing.

The basic construction of the barrel is similar to that of the previoustype, with barrel 16-1 cutting edge 16-2, segmented catcher 16-3, liner16-4 and washer 16-5.

In this case the floating piston 16-6 is held in place by tension strap16-7, which could be a cable or chain, attached by pins 16-8 and 16-9.

In operation, drill water pressure is applied to extend the piston tothe position shown on the left side view. The water in the barrel andsealed drill string is then locked off with suitable valving, not shown,to hold the piston in the extended position as the barrel is pushed tothe required sampling depth.

Once the sampling depth is reached, the top of the piston is connectedto the hydraulic tether and the barrel extended as with the previousscheme to the position on the right hand view where the piston is nearthe top of the barrel.

The sample is extracted by first removing the cutting edge and catcher,then disconnecting the strap by removal of pin and pushing out thewasher, liner, piston and sample, as before.

FIG. 17 shows a slight variation on FIG. 16 where the piston 17-1 isretained in its lower position by the use of a spring retaining ring17-2 acting against the top surface of the cutting edge. Alternatively,a groove could be provided in the barrel or liner.

This scheme has advantage in that it facilitates the fitting of a checkvalve 17-3 which will provide improved retention of the sample duringretraction and storage, but the check valve removes the possibility ofusing drill water pressure to push the piston down to its starting pointshould it be inadvertently moved out of position. The retaining ring canbe used without a check valve.

In operation, the barrel is pushed to depth as before, then connected tothe hydraulic tether and the barrel advanced. As the barrel passes overthe piston, the retaining ring will be pushed back into its groove bythe bottom edge of the liner contacting the upper chamfered face of thering.

The word ‘comprising’ as used in this description and in the claims doesnot limit the invention claimed to exclude any variants or additionswhich are obvious to the person skilled in the art and which do not havea material effect upon the invention.

Modifications and improvements to the invention will be readily apparentto those skilled in the art. Such modifications and improvements areintended to be within the scope of this invention.

The claims defining the invention are as follows:
 1. A method ofacquiring a core sample of seabed material into a core sampling tubehaving an upper end, a lower open end and a substantially cylindricalchamber extending therebetween, comprising the steps of: (a) urging thecore sampling tube into the seabed and (b) simultaneously withdrawingfluid from the upper end of the core sampling tube at a rate sufficientto cause the seabed material to be drawn into the core tube atsubstantially the same rate as the core tube penetrates the seabed. 2.The method of claim 1, wherein step (b) comprises withdrawing the fluidthrough a conduit means having two ends and connected at one end to thecore sampling tube and connected at the other end to a remote means forwithdrawing fluid.
 3. The method of claim 1, wherein steps (a) and (b)are performed by a combination of remotely coordinated hydraulic fluidpower means.
 4. The method of claim 3 wherein the coordination of thehydraulic fluid power means comprises the steps of pumping hydraulicfluid into a first hydraulic means to urge the core sampling tube intothe seabed and simultaneously pumping hydraulic fluid into a secondhydraulic means to withdraw fluid from the upper end of the coresampling tube.
 5. The method of claim 1, wherein the core sampling tubefurther has a piston sealingly engaging and movable within thecylindrical chamber above the seabed material entering the core tube,and in step (b) fluid is withdrawn from above the piston such that thepiston is maintained substantially stationary.
 6. A core sampling tubefor acquiring a core sample of seabed material comprising: (a) a corebarrel having an upper end with a fluid inlet/outlet; (b) an open lowerend; and (c) substantially cylindrical chamber extending therebetween toreceive seabed material,  wherein in use, the urging of the coresampling tube into the seabed and simultaneous withdrawing of fluid fromthe upper end of the core sampling tube is at a rate sufficient to causethe seabed material to be drawn into the core tube at substantially thesame rate as the core tube penetrates the seabed.
 7. The core samplingtube of claim 6 further comprising a piston sealingly engaging thecylindrical chamber and movable axially within the cylindrical chamberin response to fluid flow through the inlet/outlet.
 8. The core samplingtube of claim 6 further comprising a remote means for withdrawing fluid.9. The core sampling tube of claim 8, wherein the remote means isconnected to the core sampling tube by an intermediate conduitconnectable between the core sampling tube and the remote means.
 10. Thecore sampling tube of claim 9 further comprising an adaptation at theupper end to provide sealing means to permit a leak free connection tothe intermediate conduit.
 11. A seabed coring system for acquiring acore sample of seabed material, the system comprising: (a) a coresampling tube according to claim 6; (b) first hydraulic fluid powermeans to urge the core sampling tube into the seabed; (c) secondhydraulic fluid power means to withdraw fluid from the core samplingtube above the seabed material; and (d) first conduit means connectedbetween the core sampling tube and the second hydraulic fluid powermeans;  wherein the first hydraulic fluid power means and the secondhydraulic fluid power means are coordinated such that the seabedmaterial will enter the core sampling tube at substantially the samerate as the core tube penetrates the seabed.
 12. The seabed coringsystem according to claim 11 further comprising a piston sealinglyengaging and movable within the cylindrical chamber of the core samplingtube above the seabed material entering the core tube.
 13. The seabedcoring system according to claim 11 wherein the first hydraulic fluidpower means comprises a substantially cylindrical chamber, a pistonsealingly engaging the cylindrical chamber and movable axially withinthe cylindrical chamber to define a first chamber and a second chamber,and a piston rod connected to the piston and extending through and fromthe second chamber so that a selective application of hydraulic pressureto the first chamber will urge the core sampling tube into the seabed.14. The seabed coring system according to claim 11 wherein the secondhydraulic fluid power means comprises: (a) a first sub hydraulic meansincluding a substantially cylindrical chamber, a piston sealinglyengaging the cylindrical chamber and movable axially within thecylindrical chamber to define a third chamber and a fourth chamber, apiston rod connected to the piston at one end thereof and extendingthrough the fourth chamber. (b) a second sub hydraulic means comprisinga substantially cylindrical chamber, a piston sealingly engaging thecylindrical chamber and movable axially within the cylindrical chamberto define a fifth chamber, the piston rod of the first sub hydraulicmeans having its other end connected to the piston; and (c) secondconduit means connected between the second chamber of the firsthydraulic means and the fourth chamber of the first sub hydraulic means, wherein, as the core sampling tube is urged into the seabed by thefirst hydraulic fluid power means, hydraulic fluid is passed from thesecond chamber of the first hydraulic fluid power means into the fourthchamber of the first sub hydraulic means via the second conduit means tomove the piston therein which in turn draws the piston of the second subhydraulic means away from the first conduit means to cause thewithdrawal of fluid from the core sampling tube.
 15. The seabed coringsystem according to claim 14 wherein the first conduit means consists inpart of at least one hose with high collapse capability.
 16. The seabedcoring system according to claim 14 wherein the first conduit meansconsists in part of at least one drill rod with sealing means to providea leak free joint between the drill rod and any preceding drill rod.